The Pachmarhi Sandstones - A Statistical Study of Size Analysis

The Pachmarhi formation is exposed in a lensoid outcrop on the northern slopes of the Satpuras. At Pachmarhi proper, the formation is arenaceous and consists of crossbedded sandstones, with intervening pebbly layers, dipping mainly towards the north. About forty samples have been mechanically analysed out of a total of seventy samples collected from three sections of 2000, 900 and 400 feet height. The statistical analysis has yielded certain descriptive measures according to the method of Inman. These measures, obtained on a phi scale, have been graphically analysed to note vertical trends of'central tendency, sorting, skewness and kurtosis in the three sections. Further, some fifteen selected sandstones have been analysed for their moment measures. These moment measures have been plotted on Friedman's graphical representations to find that the sands are very coarse grained and the points for most of the analysed samples fall within an area delimited for sands brought by currents as strong as those of rivers. The C M pattern suggests a winnowed beach sand and is comparable in appearance to one obtained in the case of marginal shelf Red Sea sediments by R. Pass ega.

The original configuration of the basin is supposed to be such that its floor was initially inclined 50 to 60 towards the north, and the rivers and other streams brought materials of gravel to fine clay grade which were spread on the floor of the basin by local currents. Each coset with 6'-10' thickness bounded by pebbly layers has cyclic frequency due to a continued subsidence of the basin along with deposition.